Play it safe and go with the lower concentration. The packaging even states that first-time users should start with 5% in order to build up skin tolerance, and use 10% afterwards.
Do not think you can buy 10% and mix it with something else to dilute it and achieve a lower concentration. AHA products are finicky things and are formulated to sit at a certain pH to be effective. Mess with that and you may not see any results at all.
Patch test! Some stinging/tingling with AHAs is common, but be on the lookout for redness or other signs of irritation. If it's all good, use infrequently and slowly build up your tolerance. Try once a week or once every couple days and work up from there. Too much too soon, and your skin can get pretty irritated. Even though my skin is fairly tolerant now, even I got some irritation when I got greedy and tried to use it twice daily.
Wear sunscreen daily. I mean, I hope you're doing this anyway, but especially with AHAs because they increase sensitivity to sun.
Be mindful of the timing of application. If I apply something else on first, I give that 5-10 minutes before I apply the Glycolic Renewal, then wait 5-10 minutes before applying the next step. This goes back to the pH thing: I want things to have time to absorb and avoid inadvertently mixing product and changing the pH.